1. Field of the Invention
This invention relates to a semiconducter memory circuit, capable of changing the numbers of bits and words thereof, and more specifically to a multifunctional and general purpose semiconductor memory circuit which permits a user to electrically program an arbitrary logic circuit at his hand and which is most preferable for use in a programmable logic device.
2. Description of the Prior Art
There is a conventionally known programmable logic device (hereinafter simply referred to as a PLD), that is an integrated circuit with which a user can construct an arbitrary logic circuit at his hand.
The PLD primarily includes a programmable logic element (hereinafter simply referred to as a PLE) which is configurable to establish a user's own logic, a circuit function-defining memory cell for defining the logic function of the PLE and a connection relationship among internal wirings, a programmable input/output block (hereinafter simply referred to as an IOB) for interfacing between external device packaging pins and an internal logic circuit (i.e., the PLE), and a programmable wiring for establishing a wiring path to connect input/ouput signals into/from the IOB and the PLE to a desired network.
A semiconductor memory circuit for use in such a PLD is desirably variable in the numbers of bits and words meeting a user's wishes. However, such a prior memory circuit comprised of a gate array is impossible to change those numbers of bits and words after the wiring, even if they are variable upon wiring. To solve this, the memory circuit is obliged to have larger numbers of such bits and words, allowing unnecessary memory cells to be increased, followed by reduction of the degree of integration of the PLD and lowering of the utility of the memory circuit.